We have observed that following total body electron beam therapy (3600 rads delivered over 8 weeks), patients with MF have entered either into a "recapitulative" or "tolerant phase" of the disease. Because of this phenomenon, it is possible to study mycosis fungoides by some new approaches. The objectives of this research are as follows: To determine (1) the ultrastructure of all the abnormal cells in the cutaneous lesions of mycosis fungoides; (2) to investigate the problem of the site of origin of the abnormal cells in mycosis fungoides by determining whether they appear in the skin before being detected in the peripheral blood or vice versa; (3) to study the responses of patients to a variety of cell-mediated hypersensitivity tests in relation to the stage and activity of the disease, (4) to calculate mitotic and labeling indices for the abnormal cells in the skin and peripheral blood in relation to the stage of disease and the response to electron beam therapy and (5) to determine chromosomal karyotypes before, and after electron beam therapy to determine whether they remain constant or change thereby probing the question of whether mycosis fungoides begins as a benign inflammatory disease or as a neoplastic one.